Experimental Research On Multi-photon Quantum Information Processing With New Type Of Ultra-bright Entangled-photon Source

There were different understandings and debates on quantum theory through the century since quantum mechanics was born in the beginning of the twentieth century. But with the development of the quantum mechanics recently, it is clear that the quantum mechanics is an authorized theory in our world. By combining information science and quantum mechanics in ninety years of twentieth century, quantum information science develops faster and faster. Quantum information science mainly includes quantum communication and quantum computation. The processes of preparation, operation, transmission, storage and measurement of quantum states are the fundamental question of quantum information. There are many systems can be used to study the quantum information process, such as linear optics, quantum dot, NMR system, cavity QED, atom system et al. Linear optical is a mature technology for completing quantum information process for two reasons. First, the modern communication is based on fiber net, and photons are the fastest natural flying bit. So the research of quantum communication is most based on optics system. Second, Knill, Laflamme, and Milburn publish a paper which shows the probability of building a scalable quantum computer by two-qubit operations in linear optical circuits with ancillary photons. Their work inspires the experimental realization of quantum computation by linear optics.Bipartite photons entanglement state is the simplest quantum system which we usually produce by SPDC process. Bipartite photons entanglement state provides us a probability to access multi-photon entangled state preparation. This dissertation starts with the improvement of the preparation of a bipartite polarization-entangled photon source pumped by a femtosecond pulse laser, focuses on the preparation, measurement and interference of entanglement state. Our main results are follows: 1. QCCS experiment by using eight-photon entangled stateMulti-partite entangled states are important for developing studies on quantum networking and quantum computation. To date, the largest particle number that scientists have successfully manipulated in multi-partite experiments has been eight trapped ions. As to photons, due to the poor photon-pair production rat in spontaneous parametric down-conversion (SPDC), the largest photon number that has been successfully manipulated in experiments is six. We greatly improve our entanglement state source and consequently, with only 400-mW ultraviolet light as pump, for the first time we experimentally prepare an eight-photon GHZ-type entangled state and demonstrate an eight-part quantum communication complexity scenario(QCCS). Our experiment results surpass the classic limit, which proves that the eight-photon entangled state we prepared violates the Bell inequality.2. Experimental demonstration of decoherence induced spontaneous symmetry breakingComplicated quantum systems are inevitably disturbed by external environment in real world. This has become the main obstacle in the progress of realizing large scale quantum information processing. The process that entanglement is destroyed by the interaction between quantum systems and the environment is essentially a"decoherence"process. And more recently, an interesting theoretical result about decoherence induced spontaneous symmetry breaking on entangled state is presented by Karpat and Gedik. To demonstrate their theory, we experimentally investigate the variations of exchange symmetry properties of the four Bell states in an exchange-symmetric pure dephasing process with two-photon system. Experiment results show that under such a exchange-symmetric local noise Hamiltonian, the exchange symmetry property remains unchanged for two of the three symmetric Bell state. For the anti-symmetric Bell state, the exchange symmetry property increases and achieves a maximum value of 0.5 at the asymptotic limit. But for the Bell state ,the exchange symmetry property broke and survived with a probability of 0.5 at the asymptotic limit, which demonstrates a kind of spontaneous symmetry breaking phenomena induced by decoherence.3. An experiment about"Largeness"Quantum coherence is the most distinct feature of quantum mechanics. But inevitable decoherence processes finally destroy it and make the"Schrodinger's cat"invisible in our classical world. In this"quantum-to-classical transition", the so-called"largeness"plays a critical role. We experimentally study the largeness phenomena in bipartite entanglement decay process through a depolarizing channel with two-photon entangled states generated from spontaneous parametric down-conversion source. Our experiment demonstrates how the speed of entanglement decay and the time that"entanglement sudden death"happens depend on the size of system exposed to the environment noise.